example refactor snowflake module system

nix/snow/flake/modules/nixosConfigurations.nix

@@ -0,0 +1,36 @@
+{ lib, ... }:
+let
+  inherit (lib)
+    mkOption
+    types
+    literalExpression
+    ;
+in
+{
+  options = {
+    flake.nixosConfigurations = mkOption {
+      type = types.lazyAttrsOf types.raw;
+      default = { };
+      description = ''
+        Instantiated NixOS configurations. Used by `nixos-rebuild`.
+
+        `nixosConfigurations` is for specific machines. If you want to expose
+        reusable configurations, add them to [`nixosModules`](#opt-flake.nixosModules)
+        in the form of modules (no `lib.nixosSystem`), so that you can reference
+        them in this or another flake's `nixosConfigurations`.
+      '';
+      example = literalExpression ''
+        {
+          my-machine = inputs.nixpkgs.lib.nixosSystem {
+            # system is not needed with freshly generated hardware-configuration.nix
+            # system = "x86_64-linux";  # or set nixpkgs.hostPlatform in a module.
+            modules = [
+              ./my-machine/nixos-configuration.nix
+              config.nixosModules.my-module
+            ];
+          };
+        }
+      '';
+    };
+  };
+}